A transmission, in which a geared transmission unit comprising gear pairs, and a switching mechanism selectively allowing and disabling the gear pairs to transmit a torque between an input shaft and an output shaft, is connected in series with a mechanism capable of varying a speed change ratio thereof continuously is known in the prior art. For example, the transmission of this kind is disclosed in Japanese Patent Laid-Open No. 2007-177925 and Japanese Patent Laid-Open No. 2003-113934. Specifically, according to the transmission taught by the above-listed documents, gear pairs for setting gear stages are arranged on two intermediate shafts and an output shaft, and a differential mechanism having a motor therein is arranged between the intermediate shafts. If a speed change operation is carried out in the transmission thus structured by merely switching the gear pair for transmitting the torque, the speed change ratio has to be changed stepwise. Therefore, according to the teachings of the above-mentioned documents, rotational speeds of members to be connected with each other by a switching mechanism are almost synchronized with each other when carrying out a speed change operation, by varying the rotational speed of the gear pair to transmit the torque after the speed change using the motor. Consequently, the speed change ratio can be varied smoothly.
Another example of the transmission of this kind is disclosed in Japanese Patent Laid-Open No. 2004-150627. According to the hybrid transmission taught by Japanese Patent Laid-Open No. 2004-150627, a gear mechanism is formed by a plurality of planetary gear mechanisms, and the gear mechanism is configured to set a plurality of gear stages by connecting rotational elements of the planetary gear mechanisms, by changing connections of the rotational elements, and by selectively halting the rotational elements. In addition, a planetary gear type power splitting mechanism adapted to distribute a power of an engine to a motor and to the gear mechanism is arranged in an input side of the aforementioned gear mechanism. Therefore, the hybrid transmission taught by Japanese Patent Laid-Open No. 2004-150627 is capable of varying an input speed of the geared transmission mechanism arbitrarily by controlling a speed of the motor. For this reason, the hybrid transmission taught by Japanese Patent Laid-Open No. 2004-150627 is capable of carrying out a so-called “synchronous control” by the motor on the occasion of carrying out a speed change by the geared transmission mechanism.
In addition, an example of a mechanism capable of varying a speed change ratio continuously is disclosed in Japanese Patent Laid-Open No. 6-257655. The mechanism taught by Japanese Patent Laid-Open No. 6-257655 comprises a ball contacted with an outer face of a V-pulley, and first and second rotation means whose inverted conical surfaces are contacted with the ball to hold the ball. According to teachings of Japanese Patent Laid-Open No. 6-257655, a speed change ratio is varied by axially moving the V-pulley functioning as a driving member, one of the rotation means being idled, and the other rotation means functioning as an output member, thereby changing contact points between those members and the ball.
Thus, according to the transmissions taught by Japanese Patent Laid-Open No. 2007-177925, Japanese Patent Laid-Open No. 2003-113934, and Japanese Patent Laid-Open No. 2004-150627, the speed change operation of the geared transmission mechanism is carried out stepwise. However, the transmissions taught by the above-mentioned documents are capable of setting a speed change ratio between the gear stages of the geared transmission mechanism by controlling the motor while synchronizing rotational speeds during the speed change operation. Therefore, those transmissions are capable of carrying out the speed change operation smoothly, that is, capable of carrying out the speed change operation substantially continuously. However, according to the transmissions taught by the above-mentioned documents, the rotational speeds are controlled by the motor to be synchronized. That is, the motor is required other than the engine. Therefore, the transmission has to be enlarged entirely, and energy efficiency thereof may be deteriorated.
Meanwhile, the mechanism taught by Japanese Patent Laid-Open No. 6-257655 is capable of varying a ratio between a rotational speed of the input member and a rotational speed of the output member continuously. However, V-pulley and the rotation means have to be moved in the axial direction for this purpose, and a specific control is required to thus carry out a speed change operation. Therefore, devices applicable to the mechanism taught by Japanese Patent Laid-Open No. 6-257655 have to be limited. In addition, one of the rotation means has to be idled, therefore, the mechanism taught by Japanese Patent Laid-Open No. 6-257655 cannot be configured as a transmission comprising three rotary elements.